This volume contains part of the work of Coetus VII of the Consilium ad exsequendam Constitutionem de Sacra Liturgia, on the post-Vatican II reform of the hymns of the Divine Office (Roman Breviary). It was published in 1968 for the purposes of both experimental usage and wide consultation. A number of significant differences exist between this interim book and the eventual outcome of the reform as contained in the 1972 Liturgia Horarum.
The Consilium was the body responsible for carrying out the post-Vatican II liturgical reforms according to Sacrosanctum Concilium, the conciliar constitution on the liturgy. It was split into various different coetus (groups), each responsible for a different part of the reforms. Coetus VII had responsibility for reforming the hymns of the Roman Breviary (Breviarium Romanum), also known as the Divine Office (Divinum Officium).